Source measure units (SMU) are used to make precision measurements in many fields, including the testing of semiconductor products. For example, U.S. Pat. No. 5,039,934 describes one such device and range-changing in such a device is described in U.S. Pat. No. 5,144,154, both of which are incorporated herein by reference in their entireties. Typical SMU designs include a voltage or current source with integrated voltage and current measurement capabilities. A device under test (DUT) is coupled to the SMU and is then stimulated with either the voltage or current source. A typical SMU will operate properly so long as the load impedance matches the selected SMU sourcing method. If the SMU is configured to source voltage and measure current (SVMI) then a high impedance load is desirable. If the SMU is configured to source current and measure voltage (SIMV), a low impedance load is desirable. In practice, the load impedance may not match the SMU source impedance. This can lead to ringing and other transient problems. Additional problems surface when the load impedance changes with frequency, e.g., the correct sourcing method for DC is not the correct method at higher frequencies. Existing SMU designs do not change their character to adapt to changing DUT impedances with frequency. It would be desirable to provide improved source impedance ranging capabilities to address these problems.